Electrically conducting organic polymers have been of scientific and technological interest since the late 1970's. These relatively new materials exhibit the electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers. Technological application of these polymers are beginning to emerge. Herein we describe electrically conducting polyparaphenylenevinylenes, polyanilines, polythiophenes, polyfuranes, polypyrroles, polyselenophene, poly-p-phenylene sulfides, polyacetylenes formed from soluble precursors, combinations thereof and blends thereof with other polymers.
Selected regions of films of the materials of the present invention can be made conducting upon the selective exposure to a source of energy. These materials can be used to make a patternable conductive resist material and can be used as an electrostatic discharge layer, or as an electromagnetic interference (EMI) layer on the surface of a substrate. Because of the electrostatic properties of these materials they can be used as an electron beam resist which acts as its own discharge layer. Moreover, since the materials of the present invention can be readily removed from a surface on which they are deposited, they can be used as a removable electrostatic discharge layer disposed on a dielectric surface under electron beam microscopic examination, for example in a scanning electron microscope (SEM). A removable electrostatic discharge layer permits SEM analysis without destroying the sample.
The article entitled "Photoinitiated Doping of Polyacetylene", T. Clarke et al., J.C.P. Chem Comm., 1981, p. 384, describes insoluble polyacetylene selectively doped with proton acids. The polyacetylene is impregnated with one of the group of triarylsulfonium and diaryliodonium which are innert to the polyacetylene but which on irradiation with a uv light undergo a photochemical reaction leading to the doping of the polyacetylene to make it conductive. The impregnated film can be selectively exposed to light through a mask to selectively dope selected regions of the polymer. Polyacetylene is not suitable, as a resist material as are the materials of the present invention, since the polyacetylene is not soluble. For a polymer to be suitable as a resist material it must be capable of having a preselected pattern formed therein, for example on exposure to radiation, with either the exposed or unexposed region being soluble in a solvent while the other of the exposed or unexposed region is insoluble in the solvent.
The article entitled "Photochemically Doped Polypyrrole" of S. Pitchumani et al., J. Chem. Soc., Chem. Commun., 1983, p. 809, describes photochemically doped polypyrrole using as photochemical dopant diphenyliodoniumhexafluoroarsenate. Doping was accomplished by immersion of the polypyrrole substrate in a solution containing the photochemical dopant in methylene dichloride followed by irradiation with a mercury arc. Polypyrrole is not suitable for a photoresist material since the polypyrrole is insoluble.
In both the article of Clarke et al. and the article of Pitchumani et al. an insoluble solid polymer is immersed in a solution containing a dopant material. The dopant material and the solvent are absorbed into the surface of the material. In both cases the solvent in combination with the dopant is exposed to light to render the irradiated part of the polymer electrically conducting. Because of the non-soluble nature of the polymers, the dopants can only be impregnated or absorbed into the surface of the polymer film.
The articles entitled "Polyaniline; Processability From Aqueous Solutions and Effective Water Vapor on Conductivity" to M. Angelopoulous et al., Synthetic Metals, 21 (1987) pp. 21-30, and the article entitled "Polyaniline: Solutions, Films, and Oxidation State" to M. Angelopoulous et al., Mol. Cryst. Liq. Cryst 160-151 (1988), describe a chemically synthesized emeraldine base form of polyaniline which is soluble in various solvents. The emeraldine base is doped by reacting, the emeraldine powder or film with aqueous acid solution for several hours, for example, aqueous acetic acid or aqueous HCl. In contradistinction, according to the present invention where conductive polymeric materials are used as a resist material the dopant reagent and polymer are mixed in a solvent which is thereafter dried to remove the solvents to form a solid solution of the dopant reagent and the polymer. The solid solution is then selectively exposed to energy, for example, electromagnetic radiation, heat or an electron beam, which causes the reagent to decompose to dope those regions of the polymer which are exposed to the energy forming a conductive polymer in exposed regions. In the exposed region the polymer is rendered insoluble and in the unexposed regions the polymer is soluble and can thereby be removed to act as a negative photoresist which is selectively electrically conducting. A resist material of this kind is particularly useful for electron beam lithographic applications since the resist material can be its own discharge layer.
One problem associated with electron beam lithography is charging of the electron beam resist. This is particularly significant in microelectronic applications. In microelectronic applications, a pattern in a dielectric layer or an electrically conducting layer, can be formed by depositing a resist material thereover. A commonly used method of selectively removing the resist material is to selectively expose the resist material to an electron beam. The resist material in the exposed region is either made soluble or insoluble upon exposure to the electron beam radiation. The solubility of the unexposed region is opposite that of the exposed region. Therefore, the exposed or the unexposed region can be removed. The resist material is typically a dielectric. When an electron beam is directed at a dielectric surface, charges from the electron beam accumulate on the surface creating an electric filed which distorts the electron beam on the surface resulting in a loss of precision and displacement errors. To avoid charging the resist, it is common practice in the art to coat the resist, prior to electron beam exposure, with a thin conducting metal layer. Most metals, e.g. Au and Pd, are difficult to remove. In some cases the metal deposition process can degrade the lithographic properties of the resist due to heat and stray radiation during deposition. The polymer discharge layers of the present invention can be deposited by a simple spin coat process, whereas a metal cannot.
According to one aspect of the present invention, a resist material is provided which is on selective exposure to energy, for example electromagnetic radiation, an electron beam or heat, rendered insoluble and at the same time electrically conducting. The exposed regions are insoluble and conducting and the unexposed regions are soluble and nonconducting. When the source of energy generating the pattern is an electron beam, the pattern which is conducting forms an electron discharge path preventing distortion of the writing beam. If the dielectric layer on which the resist is deposited is thin, the discharge path does not have to be grounded. This avoids the requirement of depositing a metal layer to act as a discharge layer.
The polymer materials which are made electrically conductive according to the present invention have additional utility in providing an easily processable and low cost EMI (electromagnetic interference) layer which can provide shielding of electrical components from electrical noise.
The conductive polymer materials of the present invention can also be used as an electrical discharge layer for scanning electron microscopic applications. Typically, when a sample is being analyzed under a scanning electron microscope a thin metal layer is coated onto the sample. A commonly used metal layer is gold or a mixture of gold with other metals. This thin metallic layer acts as an electrical discharge layer to prevent the accumulation of electrical charge on the surface of the sample being examined. When a metallic material is used as an electrical discharge layer it cannot be easily removed, therefore, the sample being examined must be discarded. This is particularly costly in microelectronic applications where it may be desirable to examine a semiconductor chip or semiconductor chip packaging substrate with an electron microscope where the chip or substrate is functional and useful. Depositing a thin metallic layer onto a substrate would render the chip or substrate not usable. By using the electrically conducting polymeric materials according to the present invention, a functional sample can be coated with the polymer, subjected to examination under an electron microscope and thereafter the electrically conductive polymer can be easily removed permitting electron microscopic examination of the functioning part and permitting it to be subsequently used.
It is an object of this invention to provide an electron beam resist material which functions as an electrical discharge layer.
It is another object of this invention to provide a resist material whose solubility and conductivity are dependent upon a dopant species generated by exposure of a dopant precursor to energy.
It is another object of this invention, to provide a method of forming a solid solution of a polymer which is selectively transformed to the conducting state by being selectively exposed to a source of energy.
It is another object of this invention to provide a conductive polymeric electromagnetic interference layer.
It is another object of this invention, to provide an electrical discharge layer for substrates exposed to electron beam radiation, wherein the discharge layer is removable.